Long-standing legal precedent suggests the attorney general, Lord Goldsmith, should stand aside from deciding whether Tony Blair or other senior Labour figures should be prosecuted for offering honours in return for party donations, legal experts and opposition MPs claimed yesterday.

The shadow attorney general, Dominic Grieve, renewed his attack, saying if Lord Goldsmith has a personal conflict of interest in deciding whether a prosecution would be in the public interest there was a convention that he should not become involved. "I very much hope that in this case the attorney general recognises that if there was any question of a prosecution of a senior Labour politician that he stand aside," he said.

Experts highlighted how in the 1970s, Sam Silkin, the Labour attorney general, withdrew from playing any part in the decision whether to prosecute Jeremy Thorpe, the former Liberal leader who was charged with murder and conspiracy to murder, at a time when the Labour government was keen to cooperate poltically with the Liberals. Jeffrey Jowell, professor of law at University College London, said: "If the attorney is only acting under his power of superintendance, there is precedent in the Thorpe case for his finding a way to protect himself against the perception of impartiality by, for example, appointing external counsel to advise on the matter."

At his Downing Street press conference, Mr Blair refused to discuss any aspect of the police investigation or the involvement of Lord Goldsmith, whose potential conflict of interest was first raised in the Guardian last Saturday.

Mr Blair is expected to be interviewed by the Metropolitan police by the end of the year on why Labour offered peerages to four big lenders to the party. Determined to maintain the constitutional role of the attorney general, Lord Goldsmith has said he would make up his mind in the interests of justice on the legal facts and would not be swayed by his political views. As the minister "superintending" the Crown Prosecution Service, he has the right to be consulted on difficult, sensitive and high-profile cases and the power to override decisions, though this must not be done on political grounds.

Former minister Clare Short said Lord Goldsmith should recognise his involvement would make "everything look murky. It makes our system look dirty and it would be in everyone's interest if he said 'yes, I'm too close, I'll step aside and let the process take place'."

There had been reports yesterday - rapidly denied - that the director of public prosecutions, Ken Macdonald, would quit if he believed the attorney general had improperly blocked bringing charges.

Lord Mackay, the former Tory lord chancellor, said Lord Goldsmith's personal relationship with Tony Blair might make matters difficult, "but my impression is that if you are attorney general and you're in office at the time, you can hardly escape responsibility for the prosecution".